There is a growing worldwide need to expand the amount of communication that can be carried by audio communication channels, in both radio transmission and hardwired communications systems.
One technique to make better use of the frequency bandwidth of speech and other transmission channels is to use a bandwidth compression arrangement that would allow for transmission of the information content of the speech through a channel having a bandwidth substantially narrower than that required for transmission of the speech wave itself. Most of the systems are based upon the channel vocoder of H. W. Dudley described in U.S. Pat. No. 2,151,031, issued May 21, 1939.
These prior art systems suffer from a disadvantage of producing an artificial sounding output. This is caused by the vocoder's technique of preserving only the amplitude while passing a frequency that is not the true frequency but is representative of the middle frequency of the individual channel's passband. Additionally, many of the prior art systems rely on digital transmission channels, a limitation not present in the described invention.
In addition to bandwidth compression, the present invention is equally capable of bandwidth expansion for spread spectrum transmission applications and frequency normalizing for variable speed tape scanning.